WASHINGTON (AP) — U.S. officials say a pair of Russian Su-24 attack planes buzzed a U.S. Navy destroyer in the Baltic Sea multiple times on Tuesday and Wednesday.

The Russian planes appeared to be unarmed but did not respond to attempts by the crew of the USS Donald Cook to contact them by radio during the incidents. That's according to a U.S. defense official who was not authorized to discuss details and so spoke on condition of anonymity.

The official said the commander of the USS Donald Cook deemed the Russian actions unsafe and unprofessional, and that the U.S. believed this constituted a violation of a 1970s agreement meant to prevent unsafe incidents at sea.

The officials said the Cook was operating in international waters 70 miles off the Russian enclave of Kaliningrad.